feat: add client-tools to swift SDK

[louisjoecodes]

• Adds the ability for client-tools to be called in Swift

Usage:

var clientTools = ElevenLabsSDK.ClientTools()

// Register a simple tool to greet someone
clientTools.register("say_hello") { parameters in
    let name = parameters["name"] as? String ?? "World"
    return "Hello, \(name)!"
}

let sessionConfig = ElevenLabsSDK.SessionConfig(agentId: "agent-123")

...

// Start the conversation
let conversation = try await ElevenLabsSDK.Conversation.startSession(
    config: sessionConfig,
    callbacks: callbacks,
    clientTools: clientTools // Pass the tools here
)